Electromagnetic devices, in general, have coils made of insulated conductors for creating magnetic fields by electrical currents. The electrical resistance of the conductors causes heat to be generated within the conductors and thus throughout the mass of the coils. Depending on the use, the coils may include encapsulation materials that provide thermal conductivity for removing heat generated by currents flowing in the coils and may provide rigidity for preventing their insulation abrasion. For example, such coils are used in electric motors, loud speakers, scanners, torque motors, recording heads for VCR""s, computer memories, inductors, etc.
In general, electric motors transform electrical energy into mechanical energy. Every electric motor has a rotor that in most cases includes a moving part, and has a stator. A stator is made of a magnetic material and electrical conductors for establishing and shaping magnetic fields that interact with a rotor.
Galvanometers are frequently called torque motors since they can provide large torque. Mainly, there are three types of galvanometers, that is, the moving coil or so-called D""Arsonval galvanometers, the moving iron galvanometers and the moving magnet galvanometers. While the present invention is applicable to various electric devices, it is illustrated on moving magnet torque motors that have a moving magnet rotor, i.e., the rotating armature that defines the field, and a stationary coil.
Among other parameters, torque motors are characterized by the torque to inertia ratio, which expresses the acceleration capability, the electrical time constant, which burdens the drive electronics, and the first uncontrollable resonant frequency, which limits the stability of a servo system involving the motor. Over the last two decades, improved magnetic materials have been used to improve the performance of torque motors.
The stationary coil of a stator also affects the performance of torque motors. In general, coils include conductors of different shapes, distribution, resistivity, thickness of their electrical insulation, the thermal conductivity of their insulation and their encapsulating compound. For these reasons the torque motors are greatly affected by packing efficiency of the coil conductors. The packing efficiency is defined as the ratio of the volume (or cross-sectional area allocated for the coil) to the volume (or cross-sectional area) of the conductor. The conductors are typically made of copper, but in critical applications, a lower resistance material such as silver may be used.
Different manufacturers supply magnet wires with a high quality insulation material (or coating) that is thin, has uniform high temperature properties and is resistant to abrasion. There are also magnet wires with self-adhesive coatings, whose adhesive property is promoted by heat or the application of alcohol or the like, when forming the coils.
Regular winding processes (i.e., layer winding) optimize the packing density of the wires. The theoretical packing density of round cross-section conductors and very large coils can be as high as 90%. However, most layer-wound coils rarely achieve an 85% packing density for simple shapes such as in solenoids. Most motor coils yield a density close to 70%. This density can be increased after winding by compressing the coil to deform and compact the conductors together. U.S. Pat. Nos. 3, 348,183; 3,528,171 and 4,543,708 list packing and thermal benefits obtained generally and specifically with electric motors. Square or rectangular wire cross-sectional shapes may offer higher packing density, as used in inductors and loud speakers.
Theoretically, square or rectangular shapes can be wound with a density of 100%. However, such density is only reached with extremely simple coil shapes or for hand-assembled magnets such as used for quadrupole magnetic lenses or cyclotron type applications. Complex coils are not efficiently wound with square or rectangular conductors because it is extremely difficult to control the consistency of the formation of the layers of the conductor. An additional difficulty is raised with rectangular conductors, of high aspect ratios, as they do not bend in the cross-dimension without compromising the insulation on their exterior.
The present invention relates to coils for electric devices, such as electric motors, loud speakers, scanners, recording heads for VCR""s or inductors, and to methods of making the coils. The present invention relates to torque motors such as moving magnet galvanometers and methods of making the same.
According to one aspect, an electric device employs a composite electromagnetic coil including two pancake-shaped coil parts (i.e., windings) made of a rectangular or ribbon wire, wherein the wide-dimension of the ribbon cross-section extends normal to the face of the pancake. The composite electromagnetic coil includes the two pancake coil parts pre-shaped, superposed and connected to form a single tightly packed unit.
Preferably, the electric device may be a torque motor. The torque motor may be a moving magnet galvanometer. In general, the pancakes of the composite coil are in a deformed state conforming to a predetermined space in which they are to be installed. Preferably, the pancake coil parts of the composite coil are in a deformed state conforming to the shape of the stator or rotor or both. The pancake coil parts may have an elongated form in the direction parallel to the axis of the rotor. The cross-section of the rectangular or ribbon wire is in the range from about 1 to 1 to about 1 to 6 or more. Preferably, the cross-section of the rectangular or ribbon wire is in the range from about 1 to 3 to about 1 to 5. The wire carries on its exterior an activatible adhesive that helps the fabrication process.
According to another aspect, a method for making a composite electromagnetic coil includes winding two pancake-shaped coil parts made of a rectangular or ribbon wire, wherein the wide-dimension of the ribbon cross-section extends normal to the face of the pancake. The method also includes forming each pancake coil part to a selected shape, superposing the two coil parts and electrically connecting them to form a single tightly packed unit.
Preferably, each coil part is shaped to conform to the shape of a stator or a rotor of an electric motor. The winding of each pancake coil part may include forming a flat coil elongated in the direction parallel to the axis of a rotor of a torque motor, such as a moving magnet galvanometer. The two coil parts are shaped separately prior to joining them together. The method may include potting the two coil parts together.
According to yet another aspect, a method for making a composite electromagnetic drive coil for a moving magnet galvanometer includes winding two flat pancake-shaped coil parts using a rectangular conductor having a width and a thickness, wherein each pancake-shaped coil part has the width oriented in a direction substantially normal to a face of the pancake-shaped coil part. The method includes forming each pancake-shaped coil part to a selected shape different from an initially flat shape, superposing the two coil parts; and electrically connecting the superposed coil parts to form a single tightly packed unit. The method may also include potting the superposed coil parts.
Preferably, the method includes employing one cylindrical mandrel for each of the coil parts resulting in a slightly different shape. The different shape enables optimal packing of the two coil parts. The superposing includes aligning the coil parts in an orientation that adds their magnetic fields together. The connecting is performed between inner turns of each of the coil parts to locate a connection within the bounds of the composite coil without crossing.
According to yet another aspect, an electromagnetic coil is formed from two or more windings (i.e., coil parts) of a ribbon-shaped or rectangularly-shaped conductor, and preferably an even number of the windings. Each winding can be made as a flat pancake or a two layer pancake, and then shaped to a semi-cylindrical, semi-circular, semi-elliptical or another shape. Two or more windings are superposed, one above the other, in a way that preferably adds the magnetic field. In general, the coil superposition may be used to xe2x80x9cshapexe2x80x9d the resulting magnetic field, including partial addition and subtraction.
The present methods may be used to form multi-layer coils having complex shapes, wherein the coils are formed using a ribbon-shaped or rectangularly-shaped conductor, having a moderate aspect ratio such as 3, 4 or 5 to 1. These coils can be wound and formed successfully in a very convenient and low-cost manner. The suitability of such multi-layer coils is dictated by the application, the power supply voltage, and the selected driver.
Preferably, a composite electromagnetic coil for a moving magnet galvanometer is made from two pancake ribbon coil parts. The coil parts are shaped to have a semi-cylindrical shape, are then generally aligned edge to edge, and their inner ends in the inner space of the winding is electrically connected. The two assembled coil parts are potted together. The resulting composite coil has a semi-cylindrical form complementing the form of magnetic materials of the stator and the rotor. A moving magnet galvanometer uses two such composite coils, as the drive coils, that are preferably potted together to form one unit. This unit has a relatively low ratio of insulation to conductor for a given resistance, and good radial heat conductivity.
In torque motors where exceptionally high acceleration is required, the composite coils provide for a selected magnetic field and heat conductivity that keeps the torque motor from overheating. The composite coils also maximize the conductive packing density, which is important because, all other features being equal, the power conception of such devices is directly proportional to the section or volume of conductor in the coil. The power consumption (P) depends on the following relationship:
P=I2xc2x7R=I2xc2x7xcfx81xc2x7Lxc2x7N2/A
wherein I=Current in the coil
R=Resistivity of the coil
xcfx81=Specific resistance of the material of the wire
L=Length of one turn of the conductor
N=Number of turns
A=the total cross-section of the coil
The temperature of the coil rises faster than the power increases because the resistivity of copper for instance rises with temperature. In degree centigrade it is:
RT=R20(1+0.0393xc2x7xcex94T)
The radial dimension of the coil of a moving magnet scanning galvanometer is desirably as small as possible because, as the magnetic field is determined by the diameter of the magnet-armature, the diameter of the magnet-armature determines the inertia of the armature and consequently the dynamic performance of the device.
The composite coils provide good access to both the start and the end of a coil. Furthermore, the winding method of the invention eliminates overlap of the start leads, which normally would add undesirable thickness to the winding without contributing to the magnetic field and thus decreasing the packing efficiency.
The volume efficiency of a rectangular wire is about 75%, which compares favorably with that of a round wire, which rarely reaches about 60%, while 50% may be viewed as realistic for rotors. (See, for example, xe2x80x9cElectromagnetic Devicesxe2x80x9d by Herbert C. Roters, John Wiley and Sons, Inc.) On the other hand, in coils that use small dimension wires, the volume of the insulation becomes approximately 20% of the total volume. This is representative of single-layer insulation (minimum thickness) round wires commonly used for sub-fractional electric motors or galvanometers, such as 30 gauge (0.010 inch diameter) wire. An equivalent rectangular wire with an aspect ratio of 3 or 4 to 1 is penalized by a further 3%, yielding a copper volume of about 77.5% of the total volume of the conductor.